1. Field of the Invention
The invention relates to a liquid crystal display; in particular, to an output buffer applied in a source driver of a liquid crystal display.
2. Description of the Prior Art
In recent years, with the continuous progress of display technology, various types of display apparatuses, such as a LCD display and a plasma display, are shown in the market. Because the volume of the LCD display is much smaller the conventional CRT display, the LCD display using smaller desk space is convenient for the people in modern life.
In general, the driving apparatus of the TFT-LCD display mainly includes a source driving circuit and a gate driving circuit. For the TFT-LCD display having high quality, high resolution, and low power consumption, the source driving circuit plays a very important role. Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of a circuit structure of a conventional source driver.
As shown in FIG. 1, the source driver 1 includes a receiver 10, a data register 12, a bi-directional shift register 14, a line latch 16, a level shifter 18, a digital-to-analog converter 20, and an output buffer 22. The main power domain of the source driver 1 includes digital power sources VDD, VSS and analog power sources AVDD, AGND.
The output buffer 22 of the source driver 1 is usually realized by an operational amplifier (OPAMP). For the source driver 1 of the TFT-LCD display, its visual performance is determined by the operational amplifier, especially the slew rate which is important to the TFT-LCD display applied in the high frame rate application.
Please refer to FIG. 2. FIG. 2 illustrates a circuit diagram of a conventional operational amplifier OPAMP used as the output buffer 22. As shown in FIG. 2, the circuit of the operational amplifier OPAMP includes a differential input stage DIS, a bias current source IBS, an output stage OS, and a compensation capacitor Cc. Because the operational amplifier OPAMP is operated under the analog power source, its input voltage Vin, output voltage Vout, and output stage OS will have operation voltages between AVDD and AGND.
For the operational amplifier OPAMP, the slew rate performance is determined by the bias current source IBS/the compensation capacitor Cc. Therefore, if the bias current source IBS is increased or the compensation capacitor Cc is decreased, the slew rate of the operational amplifier OPAMP can be effectively enhanced. However, the increasing of the bias current source IBS will consume more power and the decreasing of the compensation capacitor Cc will cause poor stability. The more power consumption and poor stability will largely affect the normal operation of the entire system.
Therefore, the invention provides an output buffer applied in a source driver of a liquid crystal display to solve the above-mentioned problems occurred in the prior arts.